The present invention relates to a guide rail which is one of the component parts of a linear guide apparatus, and more particularly to the guide rail for a linear guide apparatus which is improved by a heat treatment to increase the fatigue strength under repeated loads.
Conventionally, as a heat treatment to which the guide rail of a linear guide apparatus is subjected in order to increase the fatigue strength, for instance, a carburizing and hardening, an induction hardening or the like is employed ("General Catalogue CAT-1501" published by NSK Ltd., Oct 1984, p. 28).
In the heat treatment, the carburizing and hardening is performed by impregnating a surface of a low carbon steel with carbon to uniformly harden the entire surface. In this case, as shown in FIG. 6, a guide rail 1 is machined to form mounting bolt holes 4 after the guide rail 1 is subjected to carburizing and hardening. Hence, the surfaces 1S which are to be machined must partially be subjected to a carburization preventing treatment in advance.
On the other hand, in induction hardening, induction coils 2 are disposed at each position confronted with ball rolling grooves 1a and 1b formed in both side surfaces of a guide rail 1 as shown in FIG. 7. The induction coils 2 are connected to a high-frequency power source, so that the side surfaces of the guide rail which are confronted with the induction coil 2 are locally hardened. That is, high-frequency induction current flows in the surface layers of the guide rail 1 in the neighborhood of the ball rolling grooves 1a and 1b, so that those surface layers are heated with the ball rolling grooves 1a and 1b. Thereafter, those surface layers are cooled to form hardened layers 3.
However, the conventional carburizing and hardening has a disadvantage in that a superfluous carburization preventing treatment is required for portions of the guide rail 1 which must be subjected to post machining. In addition, the dimension of the guide rail as a workpiece to be treated is limited by the dimension of a given heat treatment furnace. Furthermore, carburizing and hardening requires a long period for the heat treatment.
On the other hand, in the induction hardening, both side surfaces of the guide rail are locally hardened focusing on the ball rolling grooves. Therefore, it is unnecessary to subject the guide rail to the carburization preventing treatment. In addition, since it is unnecessary to provide a heat treatment furnace, the dimension of the workpiece is not limited by that of the furnace. Furthermore, the subjecting period for the heat treatment is extremely short. Therefore, induction hardening has an advantage over carburizing and hardening.
However, conventional induction hardening for guide rails still presents problems. That is, as shown in FIG. 7, the boundaries 6 between a soft portion 5 which is not hardened and the hardened layers 3 are formed by the induction hardening so as to extend obliquely from the upper surface of the guide rail 1 to both side surfaces thereof. Therefore, the hardened layers 3 do not extend to the lower surface of the guide rail serving as a rail mounting surface.
As shown in FIG. 8, a moving body mounted on a slider 7, which moves along the guide rail 1, applies its weight as a repeated load W to the guide rail 1 through the slider 7 and rolling balls 16. The repeated load W causes a shearing force acting on the boundaries 6 to generate a crack 8 in the guide rail 1. There is a possibility that the guide rail 1 will fail due to the crack.